1. Field of the Invention
The present invention relates to an eye fundus blood flow meter for measuring the blood flow velocity, etc., in the blood vessels of the fundus.
2. Related Background Art
There is already known a laser Doppler blood flow meter capable of tracking the vessel of the fundus of the eye to be examined and measuring the absolute blood flow velocity of the tracked vessel, and there is disclosed, for example in the Japanese Patent Laid-Open Application No. 7-031596, an apparatus for irradiating the vessel of the eye fundus with tracking laser light and laser light for measuring the blood flow velocity. In such an eye fundus blood flow meter, a reflected light, based on the laser light irradiating the eye fundus vessel for measuring the blood flow velocity and undergoing a Doppler shift caused by the blood cells flowing in the vessel, is received by a photomultiplier and the blood flow velocity is determined from the Doppler shift.
In such conventional apparatus, however, since the amount of the reflected light undergoing the Doppler shift varies significantly, for example, according to the position of the blood vessel on the eye fundus, the gain of the photomultiplier constituting the photosensor has to be adjusted frequently, each time the blood vessel to be measured is changed or the entering path of the measuring light is changed. For this reason, the eye to be examined is given an excessive amount of light by the operations of the examiner.
Also in the gain adjustment of the photomultiplier, the gain is adjusted to a maximum immediately before the start of the tracking operation since the received amount of light is very weak in this state, and, when the tracking operation is started, a large amount of light enters the photomultiplier at such a maximum gain, so that the deterioration of the photosensor is accelerated.
In the field of ophthalmic inspecting apparatus for tracking the movement of the blood vessels on the fundus of the eye to be examined, there are already known, for example, an apparatus for effecting two-dimensional tracking by detecting the vessel movement in two locations as disclosed in the Japanese Patent Laid-Open Application No. 63-288133, and an apparatus for effecting one-dimensional tracking by detecting, in one location, movement perpendicular to the direction of the blood vessel, as disclosed in the Japanese Patent Laid-Open Application No. 6-503733.
In such ophthalmic inspecting apparatus, a one-dimensional CCD is employed as the photosensor means for receiving the tracking light reflected on the eye fundus, and the tracking operation is achieved by calculating the amount of deviation between the tracking center position and the image position of the blood vessel by processing a signal indicating the blood vessel image. In such operation, in order to optimize the level of the blood vessel image signal, the gain of an amplifier is electrically regulated either manually or automatically, or the gain of an image intensifier positioned in front of the one dimensional CCD is regulated in a similar manner to adjust the amount of light entering the one-dimensional CCD, in such a manner that the output signal level thereof lies within a predetermined range.
There is furthermore proposed an apparatus capable of tracking a moving object, by detecting the position thereof relative to a one-dimensional CCD and executing continuous feedback of the obtained position signal to image taking direction varying means which varies the image taking direction.
However the tracking in the above-mentioned conventional configurations, involving manual regulation of the gain of the amplifier for the one-dimensional CCD prior to the tracking operation or of the gain of the image intensifier for adjusting the light amount entering the one-dimensional CCD, is cumbersome, requiring at least two operators. Besides it takes a long time, resulting in excessive light irradiation of the examinee. Also, the tracking utilizing automatic regulation of the gain of the amplifier for the one-dimensional CCD prior to the tracking operation or of the gain of the image intensifier for adjusting the light amount entering the one-dimensional CCD, is less cumbersome in operation, but such automatic regulation is also conducted even while the examiner looks for the target position to be tracked, so that the entire operation is similarly time-taking, resulting again in the excessive light irradiation of the examinee.
Also in such conventional configurations, since the electrical regulation of the amplifier gain or the regulation of the gain of the light amount entering the one-dimensional CCD from the image intensifier is conducted in such a manner that a non-vessel light region on the eye fundus is not saturated, the signal level for fine vessels of a low contrast becomes very small, hindering a satisfactory tracking operation.
Also in the actual measuring operation, the eye to be examined is not completely still but repeats fine movements even in the state of fixed gaze and the laser light has to follow such fine movements. However, in case of tracking a fine vessel with a low contrast which is positioned close to a large vessel with a high contrast, if the latter approaches to the center of tracking, for example, by the fine movement of the eyeball, the tracking may shift to the large vessel of the high contrast.
Furthermore, in the apparatus capable of tracking a moving object, there has not been disclosed a signal processing method for extracting the position of the vessel.
In consideration of the foregoing, a first object of the present invention is to provide an eye fundus blood flow meter that does not irradiate the eye fundus for an excessively long time, thereby reducing the burden to the eye to be examined.
A second object of the present invention is to provide an eye fundus blood flow meter which does not irradiate the photosensor means for an excessively long time, thereby enabling efficient use of the photosensor means for a prolonged period.
A third object of the present invention is to provide an ophthalmic apparatus not associated with the aforementioned drawbacks and capable of promptly and automatically regulating the gain of image taking means, thereby achieving measurement with minimum light irradiation.
A fourth object of the present invention is to provide an ophthalmic inspecting apparatus not associated with the aforementioned drawbacks and capable of exact and stable tracking operation for a desired eye fundus blood vessel.